I'm fairly new to building WP sites. I'm building one now for a client, and hope to build 2 or 3 more (for different clients) over the next year.

Do the licenses stay with me? Or do I give them to the clients? If I buy the Business license, put it on a client's WP install, in a year are they going to get a message that they need to purchase another year's license? If I did that for 10 clients, that would be 10 clients each buying a business license...not a bad deal for you guys, of course, but probably not the intent. :-)

Another question: since I only see another 2 or 3 builds over the next year, do I just buy a personal license for each new build? Or should I get the business license and have some to spare? What if I only use 3 or 4 licenses over the next year? I suppose I can still install the others after the year is done, but won't get updates and such...thoughts?

The license key is unique to each site on which a product is licensed. The licenses would stay on whatever site you applied them too. However, to access updates and support, you'd have login using your information. So, while your client sites would be licensed, they would still technically be licensed to you. After one year, there would be a message in their plugin area on that plugin telling them they do not have an active license. The plugins should continue to work as intended as they are not effected in anyway by the license. However, there are periodic updates to WordPress, etc., that may create a conflict with the version of the plugin you have. In order to access the updated product, you (or your client) would need to renew your subscription.

If you purchase the unlimited package, you would have unlimited licenses for that year. After that year is up, you could continue to install the plugins to any site that you'd like, but, as you said, you would no longer have access to product updates and the support forums. If you purchase the 10 site or unlimited package, but only license 2-3 sites by the end of your year's subscription, there is no change or refund.